In the continuous casting of steel, special refractories are used to control the flow of the molten steel and protect the molten steel from oxidation as it is poured from steel ladles to tundishes and from tundishes to continuous casting molds. Such refractories includes lide gate plates or stopper rods used in molten metal flow control, various collector nozzles in ladles and tundishes, and protective ladle shrouds and submerged pouring nozzles employed to protect molten metals from oxidation. THese types of specialty refractories are subjected to severe operating conditions such as thermal shock, molten steel erosion, and slag attack.
Such specialty refractories are usually carbon-containing refractories and, more specifically, carbon-bonded refractories. They are usually composed of refractory grain such as aluminum oxide, zirconium oxide, clays, magnesium oxide, silicon carbide, silica, or other dense grain of specific mesh size; carbon from flake graphite, amorphous graphite, carbon black, coke and like forms; and a carbonaceous binder derived from pitch or resin.
Some oxidation takes place during the manufacture of steel, and considerable amounts of oxygen may dissolve in the molten metal. In the ensuing solidification of the steel during casting, much of the dissolved gas is expelled and, in the case of oxygen, it reacts with carbon to produce evolved carbon monoxide. The dispelled oxygen, carbon monoxide and other gases create undesirable porosity, cracks, and internal defects which lower the quality of the finished steel. In order to eliminate the problem of dissolved oxygen, molten steels are deoxidized or "killed" by the addition of aluminum metal, ferromanganese, or ferrosilicon. In the case of aluminum-killed steel, the aluminum reacts with dissolved oxygen or iron oxide to form finely-dispersed aluminum oxide, some of which floats into the slag above the molten steel and some of which remains as highly-dispersed micro particles in the solidified steel. During continuous casting, this extremely fine dispersed portion of alumina has a tendency to either precipitate out of the molten steel onto the cooler refractory surfaces or react and stick to the ceramic refractories that control the molten steel in its path from ladle to tundish to casting mold.
This precipitated alumina has a particular affinity to the typical carbon-bonded alumina-graphite refractories commonly utilized as ladle shrouds and submerged pouring nozzles, the latter also referred to as a subentry nozzle or simply as "SEN". The alumina will continue to build up in the interior of the SEN until the flow of molten steel is reduced to a point where the tube must be lanced open by an oxygen torch or the SEN is discarded. If oxygen lancing becomes necessary, the casting process is disrupted costing time and money, casting efficiency decreases, and the quality of the steel must be downgraded. A total alumina blockage of a SEN decreases the expected life of the refractories and is very costly to steel producers. In alumina-killed steels where high dissolved-oxygen concentrations are expected, the useful life of a submerged pouring nozzle may be limited to 2-3 ladles due to heavy alumina buildup on the interior diameter of the tube.
Heretofore, one of the solutions to this problem has been the development of an argon injected SEN, which allows high pressure argon to permeate the porous interior diameter of the nozzle during casting, thereby forming a protective layer of inert gas which hinders the bonding of the dispersedalumina to the refractory. The argon also reduces the oxygen partial pressure at the refractory-molten metal interface, again decreasing the possibility for adherence of alumina deposits. Exemplary of such is the gas permeable immersion pouring nozzle disclosed in UK patent application GB 2,111,880 A, to Gruner et al. The argon-injection technology has extended SEN life a step further at an ever increasing cost--the expense of large volumes of argon required during casting and the increased manufacturing costs of the more complex SEN-argon nozzles. It has been observed as well that the argon may introduce objectionable pinholes into the cast steel through absorption and subsequent expulsion of the gas as the molten metal solidifies.
It has also been proposed to provide a pouring nozzle with a lower melting point liner composition which prevents alumina buildup. Liner materials developed to date include the use of CAO-MgO-Al.sub.2 O.sub.3 liners, as disclosed in UK patent application GB 2,170,131 A to Tate, which develop low melting eutectics (between 1350.degree. C.-1600.degree. C.) which are washed out of the nozzle as alumina is deposited and reacts with the liner. The melting action prevents the alumina buildup and allows for the free flow of molten steel. Also reported to be effective in prevention of alumina adhesion is a sleeve of Magnesium oxide (MgO) according to UK patent application GB 2,135,918 to Rosenstock et al.
It has still further been proposed in UK patent application GB 2,110,971 A to Kurashina et al. to provide a submerged nozzle of a modifiedgeometry wherein the lower portion of the inner nozzle diameter is greater than the top, with an angled step therebetween to prevent blockage of the flow passage. The upper portion of the nozzleis comprised of alumina-graphite and the lower portion is zirconia-graphite.
The present invention provides a method of preventing Al.sub.2 O.sub.3 buildup during casting and an article of manufacture of a refractory composition that is formed as an interior liner on submerged pouring nozzles, ladle shrouds, collector nozzles and like components. The invention thus inhibits the buildup of alumina and other oxides on such specialty refractories used in the flow control and protection of molten steel during continuous casting of aluminum-killed steels and the like.
Still further,the present invention provides an interior liner composition for subentry pouring nozzles which substantially prevents alumina from adhering to the refractory by improving the nonwetting characteristics at the molten metal/nozzle interface. The invention provides a nozzle liner of a composition having similar thermal expansion properties tot he alumina-graphite and zirconia-graphite refractories presently in use so as to prevent cracking during firing in manufacture and during casting operations. Still further, the liner composition of the present invention provides superior steel erosion resistance, similar to the aluminagraphite body of existing SEN nozzles to allow for long casting sequences and low permeability to decrease the opportunity for unwanted exidation.
The present invention further provides a submerged pouring nozzle or SEN for continuous casting of killed steels which resists buildup of alumina on a level equal to or better than the current argon SEN. The SEN of the present invention includes a liner material which is simultaneously pressed and fired with the other conventional refractories comprising the nozzle. The invention thus achieves the desired goal of reducing alumina buildup to superior levels without the need for expensive inert gas hardware and the complicated and expensive gas permeable nozzles heretofore required in such practice.